Inmates fill sandbags for the construction of temporary levees in Butte LaRose, Louisiana.REUTERS

MORGANZA, Louisiana — A steel, 10-ton floodgate was slowly raised Saturday for the first time in nearly four decades, unleashing a torrent of water from the Mississippi River, away from heavily populated areas downstream.

The water spit out slowly at first, then began gushing like a waterfall as it headed to swamp as much as 3,000 square miles of Cajun countryside known for small farms and fish camps. Some places could wind up under as much as 25 feet of water.

Opening the Morganza spillway diverts water away from Baton Rouge and New Orleans, and the numerous oil refineries and chemical plants along the lower reaches of the Mississippi.

“We’re using every flood control tool we have in the system,” Army Corps of Engineers Maj. Gen. Michael Walsh said Saturday from the dry side of the spillway, before the bay was opened. The podium Walsh was standing at was expected to be under several feet of water Sunday.

The Morganza spillway is part of a system of locks and levees built following the great flood of 1927 that killed hundreds. When it opened, it was the first time three flood-control systems have been unlocked at the same time along the Mississippi River.

Earlier this month, the corps intentionally blew holes into a levee in Missouri to employ a similar cities-first strategy, and it also opened the Bonnet Carre spillway northwest of New Orleans to send water into the massive Lake Ponchatrain.

Snowmelt and heavy rain have been blamed for inflating the Mississippi, and the rising river levels have shattered records all set 70 years ago.

About 25,000 people and 11,000 structures could be in harm’s way.

In Krotz Springs, Louisiana, one of the towns in the Atchafalaya River basin bracing for floodwaters, Monita Reed, 56, recalled the last time the Morganza was opened in 1973.

“We could sit in our yard and hear the water,” she said as workers constructed a makeshift levee of sandbags and soil-filled mesh boxes in hopes of protecting the 240 homes in her subdivision.

Some people living in the threatened stretch of countryside — an area known for a drawling French dialect — have already started heading out. Reed’s family packed her furniture, clothing and pictures in a rental truck and a relative’s trailer.

“I’m just going to move and store my stuff. I’m going to stay here until they tell us to leave,” Reed said. “Hopefully, we won’t see much water and then I can move back in. “

It took about 15 minutes for the one 28-foot gate to be raised. Several hours will pass before any of the water hits sparsely populated communities. The corps planned to open one or two more gates Sunday in a painstaking process that gives residents and animals a chance to get out of the way.

The water will flow 20 miles south into the Atchafalaya Basin. From there it will roll on to Morgan City, an oil-and-seafood hub and a community of 12,000, and eventually into the Gulf of Mexico.

The Krotz Springs area was in a sliver of land about 70 miles long and 20 miles (30 kilometers) wide, north of Morgan City, and could get water in about 12 hours. The finger-shaped strip of land was expected to eventually be inundated with 10- to 20-feet (three- to six-meters of water, according to Army Corps of Engineers estimates.

The water wasn’t expected to reach Morgan City until around Tuesday.

Engineers feared that weeks of pressure on the levees could cause them to fail, swamping New Orleans under as much as 20 feet (six meters) of water in a disaster that would have been much worse than Hurricane Katrina in 2005. Even though water was being released from the river, the levees were still being put to the test for a couple of weeks.

“These levees will be under a lot of pressure for a long period of time,” said Corps Col. Ed Fleming.

Earlier, the corps blew up a levee in Missouri — inundating an estimated 200 square miles of farmland and damaging or destroying about 100 homes — to take the pressure off the levees protecting the town of Cairo, Illinois, population 2,800.

This intentional flood is more controlled, however, and residents are warned by the corps each year in written letters, reminding them of the possibility of opening the spillway, which is 4,000 feet (1,200 meters) long and has 125 bays.

The spillway, built in 1954, is part of a flood plan largely put into motion in the 1930s in the aftermath of the devastating 1927 flood.

This is the second spillway to be opened in Louisiana. About a week ago, the corps used cranes to remove some of the Bonnet Carre’s wooden barriers, sending water into the massive Lake Ponchatrain and eventually the Gulf of Mexico.

By Sunday, all 350 bays at the 7,000-foot structure were to be open. The spillways could be opened for weeks, or perhaps less, if the river flow starts to subside.

In Vicksburg, Mississippi, where five neighborhoods were underwater, a steady stream of onlookers posed for pictures on a river bluff overlooking a bridge that connects Louisiana and Mississippi. Some people posed for pictures next to a Civil War cannon while others carried Confederate battle flags being given away by a war re-enactor.

Vicksburg was the site of a pivotal 1863 Civil War battle and is home to thousands of soldier graves.

James Mims, 50, drove about an hour from Calhoun, Louisiana, with his wife, son and three grandchildren to snap a photo.

“It’s history in the making and we’re seeing it happen,” Mims said.

Members of the U.S. Army Corps of Engineers open the Morganza Spillway in Morganza, Louisiana. The move will flood acres of Cajun country but will save New Orleans. (REUTERS)